University of Lille 1 - Faculty of biology
Animal Biology
Sea star dissection
Morphology
The Asteria rubens sea star
is an Echinoderm
belonging to the class of Asteroidea.
It lives on costs
we may find it in the intertidal zone
at low tide.
It feeds chiefly on mussels
fixed on rocks.
The sea star is part of the foreshore food web
and can fall prey to other animals.
The body of the sea star is flattened,
in a shape of a star with 5 arms.
The arms merge at their base
and are attached to the flat disk.
The arms are called radius
The zones between the arms are called interradius.
This organisation is representative of the pentaradial symmetry,
characteristic of the adult echinoderms.
The sea star has two faces:
the upper face,
or aboral surface,
is convex and pigmented.
The bottom side
or oral surface,
is flattened, and clearer,
in contact with the substrate.
This surface includes the mouth,
located in the middle of the flat disk.
On the aboral surface,
at the interradius level,
we can see a whitish limestone plate,
the madreporic plate.
There are bony spines convering the body,
organized in rows.
On the aboral surface,
there is a row in the middle of each arm
and two or three lateral rows.
By turning the sea star over,
we can see a supramarginal row
and an inframarginal row
Two rows of prikly adambulacra
delimit a central furrow,
the ambulacral groove,
Every ambulacral groove has 4 rows of fleshy and movable tubes
tube feet or podia,
ended by a sucker.
At tip of each arms we can see eyespots,
sensory organs red in coulour
sensitives to the light.
Integument
In the sea star,
the body and the spines are covered
by an ciliated epidermis
The dermis,
situated beneath the epidermis,
has calcareous plates,
or ossicles
in which the spines are articulated.
The locations of ossicles
are visibles on the surface of the animal.
The integument has specialized structures,
pedicellariae,
put in the shape of a crown at the base of the spines.
In Asteroidae,
they have a form of a pincer with two jaws.
Pedicellariae are used
to clean the integument
and the defense of the animal
Organs used for the breathing,
gills or papules,
are associated with the integument.
they are between the ossicles of the skeleton.
The presence of an epithelium
ciliated on the inner side of the integument
enables the circulation of the liquid
inside the body cavity,
the coelom.
Echinoderms have a calcareous internal skeleton,
formed by ossicles.
the cross section of the arm enables us
to observe the organisation of the dermal plates.
We can see:
a dorsal ossicle
two supramarginal plates
and two inframarginal plates each having spines,
two adambulacra plates
and two ambulacral plates.
Between the ambulacral plates,
there are pores that allows the passage of the tube feet.
General anatomy
To observe the internal organisation of the sea star,
we have to cut the sea star with scissors.
Incise the integument
following the outline
to separate the upper half
from the lower half of the animal.
Identify the madreporic plate
and avoid it when you incise.
Lift the integument of the aboral surface
starting from the end of the arms
and cut carefully the underlying organs
ending by the flat disk.
The rest of the dissection will be done in the water.
In the coelom we can see gonads
and a part of the digestive system.
The digestive system
The digestive system includes the mouth,
the stomach divided in two superimposed chambers,
the pyloric ceca and the rectal ceca.
It ends by a microscopic anus,
located in the aboral surface.
The observation of the digestve system
will be done using the aboral portion.
In the central part of the disk,
we notice two rectal ceca.
Below we notice the upper chamber of the stomach,
the pyloric portion with a pentagonal shape.
5 canals start from this chamber
that extend themselves in the arms of the star.
Each pyloric canal is connected to two pyloric ceca
that take up a large part of the arms.
The pyloric ceca,
composed by dozens of diverticula on dead end
play a digestive key role
and are used to the storage of nutrients.
To observe the rest of the digestive system
we have to remove the rectal ceca,
pyloric ceca
and the pyloric stomach.
The ventral chamber or cardiac stomach,
has a shape of five lobes rose.
For the ingestion of preys,
this part of the stomach is everted.
After the external digestion,
the stomach gets back to its position
thanks to the contraction of five pairs of retractor muscles
put in the ambulacral plates.
To see the mouth,
we have to detach the retractor muscles
and remove the portion of cardiac stomach.
The mouth,
located in the middle of the flat disk,
surrounded by a membrane,
the peristomium.
Reproductive system
In Asteroidea
the sexes are separated
but there is not a sexual dimorphism.
Gonads are located in the coelom.
To observe the gonads
we have to remove the pyloric ceca,
and the upper chamber of the stomach.
Every gonad,
with a granular apsect,
is divided in deux lobes
that all extend in two adjacent arms.
The size of gonads is variable,
it depends on the state of sexual maturation of the animal.
Females and males produce gametes
through genital pores located in the interradius
at the junction with the flat disk.
The fecondation is extern.
Aquifer system
One of the characteristic of Echinoderms
is the existence of an aquifer system.
It is involve in the locomotion and the food intake.
This system have a network of conduits
filled with a liquid
whose composition is close to the sea water
The aquifer system have a madreporic plate,
the stone canal,
the ambulacral ring,
five radial canals,
and many tube feet or podia.
To observe the different parts of the aquifer system
we have to open the sea star
and remove the digestive
and reproductive system.
The madreporic plate, or madreporite,
is visible on the aboral surface of the animal.
This calcareous plate is riddle with holes
"aquifer pores"
that permit the sea water to enter in the system.
The madreporite is in connection with the madreporic canal
or stone canal.
This canal
sostained by superimposed calcareous rings,
go down toward the oral surface of the animal.
The stone canal
rejoin the ambulacral ring surrounding the mouth.
This ring is hidden
with calcareous plates of the skeleton.
On the circumference of the ambulacral ring of the star
in the interradial position,
we notice 9 vesicles with an immune function:
Tiedemann bodies.
From the aquifer ring goes in each arm
an ambulacral canal or radial canal,
located between the epidermis and the ambulacral plate.
From the radial canal goes many lateral canals.
Those are connected to the tube feet or podia.
The injection of a dye into ambulacral canal of an arm
enables us to hightlight
a large part of the aquifer system.
Podia have three parts:
ampulla or bulbous ampulla
a muscular and elongated foot
and a sucker at the end.
The podia go through the skeleton at pores level
located between the two adjacent ambucral plates.
To observe those pores
we have to scratch the polian vesicules
on a part of the arm.
The contraction of the polian vesicles
allows the turgescence of the tube feet
and the the monitoring of the sucker for the locomotion of the animal
The podia within the ambulacral groove
are protected by rows of movable ambulacral spines
and by many pedicellariae.
Radial canals
end with sensory tentacles
close to the eyespot.
Pedagogical design: Pierre-Eric Sautière Jacopo Viziolo
Technical production: Bernard Mikolajczyk
Designs: Jacopo Vizioli
Computer graphics: 3D MEDICUS
Voice over: Alain Nempont
Music; Ambient slamming bass jam, Jay Berlinsky
Gratitude: Sébastien Lefebvre, Michel Priem
Gratitude: Patrick Flammang, Michel Jangoux
SEEM; Education Service and Multimedia